The present invention relates to image processing, and in particular, to an image processing system and method enhancing image resolution and generating sub-pixel accuracy on images captured by metrology tools.
Conventionally, images obtained from a metrology tool, such as a scanning electron microscope (SEM), are used only for critical dimension (CD) measurement. Limits to application of the SEM images are due to insufficient resolution of resulting images processed by conventional methods distinguishing pixels corresponding to background and inspected object using local or global threshold filtering on optical property such as the intensity. The resolution is limited by the size of pixels represented in the images obtained thereby. For example, an image containing 500×500 pixels with a 10 μm field-of-view has a pixel representing size of 20 nm. Thus, a single pixel deviation results in a 20 nm-difference in resolution with metrology analysis.
As shown in FIG. 1, an object 10 is inspected and a SEM image 13 obtained therefrom. A contour of the object on the image is determined according to intensity information of pixels in the image. The grey-level SEM image is digitalized using threshold filtering according to a preset intensity or slope threshold. Thus, pixels corresponding to intensity values greater than the preset threshold are assigned a digitalized intensity value as 1, otherwise zero. For example, a digitalized image 15 has a rough contour as shown in FIG. 1. Because of the current incomplete image processing, applications other CD related measurement are difficult to achieve, such as measurements for line edge roughness, corner rounding, and other characteristics of the inspected object.
Hence, there is a need for an image processing system and method at the sub-pixel level providing enhance image resolution from metrology tools.